Planting, cultivation, and other crop-growing practices require precise maneuverability of tractors in narrow crop rows. Many tractors with mechanical front wheel drive (MFWD) and large front wheels suffer from poor maneuverability.
In order to assist the rear wheels, the typical tractor with MFWD includes a front axle pivoted to a forward portion of the tractor frame for oscillation about a fore-and-aft axis or an oscillation centerline. The axle includes a housing that supports differential gearing and oppositely extending axle drive shafts. At its outer opposite ends, the axle has steering knuckles that are supported by the driven front wheels and permit turning of the driven front wheels. If large wheels are used on the front axle, maximum turn is generally unduly restricted by tractor design and construction. Such a restriction may include potential abutting against the frame of the wheel or a fender mounted on the knuckle for movement with the wheel. For example, the John Deere 4755 tractor, manufactured by Deere & Company, has large front wheels that are restricted at maximum turn by the body or frame of the tractor.
Power from the engine is delivered to the front axle through a drive shaft that extends between the front axle and a rearwardly positioned transmission. The drive shaft typically is provided with a U-joint at each of its ends in order to compensate for misalignment caused by oscillation of the front axle. The space requirements stemming from inclusion of the U-joint exacerbate any restriction on clearance for the front wheels at maximum turn, particularly at maximum oscillation of the axle. Furthermore, the drive shaft may need shielding that occupies additional space and further restricts turning clearance of the wheels.
Maneuverability of tractors may be improved by increasing turning clearance of the wheels and enhancing visibility from the cab past the axle to the ground. An early design for increasing the maneuverability of MFWD tractors is shown in U.S. Pat. No. 4,225,151 wherein the steerable front wheels are provided with a large caster angle so that the wheels tilt upon turning and can tuck under the tractor frame. However, even with this tucking action, the turning radius of tractors was still limited, particularly during maximum oscillation.
Furthermore, U.S. Pat. No. 5,152,364 discloses an improved tractor design wherein an increased turning clearance is provided by mounting the engine above the axle in a spaced relationship and positioning the radiator far enough forward of the engine to define a volume into which even large wheels can be turned during steering. Nevertheless, the wheels may be restricted at maximum turn during maximum oscillation by the positioning and size of the drive shaft for the front axle and its associated shielding.
Even with the improvements which have been made in the maneuverability of work vehicles such as agricultural tractors, it is still desirable to provide more improvements in maneuverability.